Protective system



July 20, 1943. s 5 CRARY 2,324,825

PROTECTIVE SYSTEM Filed NOV. 13, 1941 Inventor" I Selden B. Cr'ar'y.

Patented July 20, 1943 PROTECTIVE SYSTEM Selden BQCrary, Schenectady, N.Y., assignor to General Electric Company; a corporation of New YorkApplication November 13, 1941, Serial No. 418,916

8 Claims.

My invention relates to protective systems and more particularly tosystems for the protection of synchronous dynamoelectric machinesagainst internal faults. Specifically, my invention is an improvement onUnited States Letters Patent 1,867,416 and 1,867,417, granted July 12,1932, upon applications of E. G. Merrick and assigned to the sameassignee as the present application.

Internal faults and particularly turn-to-turn faults in the armaturewinding which is usually the stator of synchronous dynamoelectricmachines may often develop into serious faults unless removed in theirincipient stages. Differential protective schemes have been used but theunbalance under external fault conditions in such machines is of such amagnitude that the sensitivity of such a system must be considerablyreduced, whereby it is unable to protect against many incipient faults.The protective scheme suggested in the above-mentioned United Statespatents detects an internal fault, such as a turnto-turn fault on thearmature or stator winding of a synchronous dynamoelectric machine inresponse to the alternating-current voltage induced in a small portionof the direct-current field winding which is usually the rotor windingof a synchronous dynamoelectrlc machine. The magnitude of thealternating-current voltage appearing across thi small portion of thefield winding relative to the alternating-current voltage appearingacross another portion of the field winding, such as the total fieldwinding, as a result of the voltage induced by the movement of the fieldwinding relative to the other winding of the synchronous dynamoelectricmachine is utilized to operate an electroresponsive device or protectiverelay. As long as there are no dissymmetries such as non-uniform air gapOr short-circuited turns in the field or rotor winding, the protectiveschemes covered by the above-mentioned Letters Patent operatesatisfactorily. However, I have found that, when a shorted turn existson the field or rotor winding of a synchronous dynamoelectric machine,very high voltages would appear across the portion of the field winding,which includes the shorted turn upon the occurrence of an external faultand would cause undesirable isolation of the dynamoelectric machine ifthe above-mentioned protective schemes were used since a correspondingincrease of potential across the total field winding does not occur.

Tests have shown and it is generally accepted that many synchronousdynamoelectric machines operate with field turns which are shortcircuitedwhen the turn-to-turn voltages increase above normal.Furthermore, other dissymmetries such as unequal air gaps and the like'may cause undesirable operation of the prior art protective schemes.Consequently, for such applications, these prior art protective schemesare unsatisfactory since the protective system may operate on externalfaults even though no internal faults on the armature or stator windingof the synchronous dynamoelectric machine exist.

Accordingly, it is an object of my invention to provide a new andimproved protective system for dynamoelectric machines whereby isolationof the dynamoelectric machine is obtained upon internal faults withoutfalse operation in the event of external faults.

' t is another object of my invention to provide a new and improvedprotective system for internal faults of dynamoelectric machines whichwill operate to isolate the dynamoelectric machine with more consistentspeed of operation than was heretofore possible.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to theaccompanying drawing in which the single figure thereof diagrammaticallyillustrates a protectivearrangement embodying my invention.

Referring now to the drawing, I have shown, for the purpose ofillustrating my invention, a dynarnoelectric machine it having tworelatively movable windings H and it which form respectively thearmature and field windings of a synchronous machine, such as agenerator, motor, or synchronous condenser. Since the armature windingll of a synchronou dynamoelectric machine is normally the stator windingenergized from a high-voltage source of alternating current, I haveillustrated this winding as energized from a suitable bus I3 through acircuit-controlling means, such as circuit interrupter H, which isillustrated as of the latched closed type provided with a trip coil I5and an a switch I 6 which is closed when the circuit breaker is closedand open when the circuit breaker is open. It should be understood that,in so far as my invention is concerned, it is immaterial which of thewindings H and I2 is the rotor or stator winding but, since the moreusual case is the one in which the armature is the stator, I haveillustrated the field winding l2 as the moving winding or rotor and itis accordingly provided with the necessary slip rings I1 and I8 whichare con nected to a suitableexciter unit 19 through a latched closedcircuit breaker 2| having a trip coil 22 and an "a switch 23. It shouldbe understood that the field winding I2 may involve any number of polessuch as are commonly used in synchronous dynamoelectric machines and,for the purpose of illustration, the field winding I! ha been shown assubdivided into four sections to indicate a field winding having fourpoles.

In order to indicate the presence of an internal fault on the armatureor stator winding H which fault in many cases is not discernible by theusual method of differential protection, I provide, as in theabove-mentioned Merrick patents, a third slip ring 24. Although I havedisclosed 1 the slip ring 24 as connected to the midpoint 25 of thefield winding l2 to obtain certain advantages enumerated hereinafter, itshould be understood that my invention is not limited to using exactlyone-half of the winding but any other predetermined portion of thewinding might be utilized. However, by connecting slip ring 24 to themidpoint 25 of the field winding I2, as illustrated, rather than toinclude only a small portion of the field winding between slip rings lland 24, a more consistent speed of operation of the protective systemmay be obtained since the potential across slip rings ill and 24 will bemore or less independent of the relative location of the internal faultwith respect to the tapped winding section as contrasted witharrangements where only a small portion of the field winding isconnected between thes slip rings.

In order to isolate dynamoelectric machine it upon the occurrence of aninternal fault in armature winding ii, for example, I provide anelectroresponsive device generally indicated at; 26 which is arranged tooperate a contact-controlling member 27 for bridging contacts 23 toenergize the trip coils l and 22 of circuit breakers l4 and 2|,respectively. Electroresponsive device 26 may comprise any of thewell-known types of relays although I have preferred to illustrate it asof the induction disk type having a plurality of disks 29 and 38 mountedon a common shaft ill for controlling contact-controlling member 2?which is normally biased to the open position indicated in the drawingby spring means 32.

In order that electroresponsive device 26 is responsive to internalfaults on the armature winding it of dynamoelectric machine it, an.operating torque is applied to shaft St to cause rotation thereof in thedirection indicated by the arrow associated with disk 2a. This isaccomplished by means of a motor element 35 associated with disk 29which is energized with a potential obtained across slip rings ill and2d through a potential transformer 35 which is provided with an air gapso that energization of motor element 3d may should be understood,however, that in large dy-v namoelectric machines, such a balancingimpedance 31 may not be required in view of the fact that the unbalancedcurrent due to the energization of primary winding 36 of potentialtransformer 35 may be a negligible portion of the induced alternatingfield current flowing in field winding l2. I

In order to prevent motor element 34 from causing contact-controllingmember 21 to bridge contacts 28 on a fault condition external of dyna ondisk 30. Motor element 38 is preferably energized in accordance with theinducedalternating field current. through a current transformer 39provided with a suitable air gap so that motor element 38 is notappreciably affected by the div rect-current component of the fieldcurrent.

With the arrangement just described, the force for causingcontact-controlling member 21 to bridge contacts 28 is proportional tothe square of the induced alternating current voltag across half of thefield winding 82 minus K times the square of the induced alternatingfield. current minus a constant. This may be expressed mathematically asfollows:

where F is the contact-closing force or torque applied to shaft 3i, E isthe efiective A. C. component of induced voltage appearing acrossonehalf of the field winding or across slip rings ii be obtained inresponse to the induced alternat- I ing current in field winding l2without appreci= ably saturating the core thereof by the direct our- 1rent flowing in the field winding. In order to balance the smallcomponent of direct current normally fiowing in the primary winding 36of potential transformer 35, a variable impedance, such as adjustableresistor 37, may be connected across slip rings I8 and 24. By properlyadjusting this resistance, no unbalance in the normal energization ofthe two halves of field winding l2 will'result by virtue of theenergization of motor element 34 associated with induction disk 29. It

and M for producing the operating torque for electroresponsive device26, I is the effective component of alternating field current forproducing the restraining torque on electroresponsive device 26, K is apredetermined constant to allow for dissetry in the windings of thedynamoelectric machine as well as for a possible maximum number ofshorted turns of the field winding, and K1 is the restraining torqueproduced by the spring 32. For internal short circuits such asturn-toturn faults of the armature winding ll of dynamoelectric machineill, a high operating torque will be obtained for motor element 3% whilea small current restraint will be obtained for motor element 38. Forexternal short circuits, on the other hand, the current restraint willbe large, thus preventing operation even though the operating voltage ishigh enough to cause operation thereof without such restraint whereby adissymmetry such as a shorted field turn combined with an external faultwill not cause false operation of electroresponsive device 26 andundesirable operation of circuit breakers l6 and 2 i.

With the above arrangement, good relay sensitivity for internal faultsis obtained since the ratio of voltage across half of the field windingto the current for either the fundamental or second harmonic, which arethe predominant components of the induced alternating current andvoltage, is one-half of the reactance of the field winding including theeffect of mutual coupling of the armature circuit when connected to asystem. This reactance has a relatively high ohmic value compared to theapparent'reactance corresponding to the ratio of the alternating voltageand current viewed from electroresponsive device 26 due to dissymmetrysuch as shorted field a turns combined with an external fault.

Accordingly, it is unnecessary to use-only a small portion of the fieldwindings as in the prior art arrangements where ability to distinguishbetween external and internal faults could not be obtained otherwise.Also, by using half of the field winding for obtaining the operatingpotential of electroresponsive device 26, much faster operation of theprotective system will consistently result since it is unnecessary forthe machine to make substantially a full revolution before the shortedturn orinternal fault in the armature winding H becomes effective asmight be the case for a particular fault location when only a very smallportion of the field winding is tapped to obtain an operating potentialtherefrom for an electroresponsive device, such as 26.

The operation of the protective system will be obvious in view of thedetailed description included above. In the event of an internal fault,

such as a turn-to-turn fault, on armature winding l i, a highalternating-current voltage will be induced in a small part of the fieldwinding as each small part of the field winding passes the shorted turnof the armature winding ll. Since the operating potential forelectroresponsive device 36 is obtained across half of the field windingi2, the high alternating-current voltage appearing across this half ofthe field winding due to a fault on armature winding II will beindependent of the internal fault location. This is due to the fact thatthe resultant alernatingcurrent voltage across the total field windingunder all conditions is substantially zero due to the negligibleimpedance of the exciter and, consequently, the voltages appearingacross the two halves of field winding l2 are essentially equal.

Therefore it is immaterial whether the small part of the field windingin which the high voltage is induced as it passes the faulted portion ofthe armature winding is in the half of the field winding between sliprings i1 and 24 or between slip rings 24 and i8. Under such an internalWault condition, the restraining torque produced by motor element 38will be very low, and consequently, the operating torque produced bymotor 34 will predominate and contacts 28 will be bridged with theconsequent opening of circuit breakers l4 and 2|. In the event of anexternal fault, the induced alternating-current component of the fieldwinding l2 will be high to provide a high restraining torque forelectroresponsive device 26 and prevent false operation thereof eventhough a dissymmetry, such as a shorted field turn, might cause motor 34to produce a high operating torque on electro-responsive device 36.

While I have described what I at present consider the preferredembodiment of my invention, it will be obvious to those skilled in theart that various changes and modifications may be made without departingfrom my invention and I, therefore, aim in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination with a synchronous dynamoelectric machine havingrelatively movable field and armature windings, an electroresponsivedevice including means for causing operation thereof in response to aturn-to-turn fault on said armature winding connected to be energized inaccordance with the alternating potential appearing across apredetermined portion of said field winding, restraining means for saidelectroresponsive device, and means for energizing said restrainingmeans from said dynamoelectric machine to prevent faulty operation ofsaid electroresponsive device under substantially all external faultconditions even though dissymmetries exist in said dynamoelectricmachine.

2. In combination with a dynamoelectric ma.- chme having relativelymovable field and annature windings, means for controlling the circuitof one of said windings operative in response to a. turn-to-turn faulton said armature winding including operating means energized inaccordance with the alternating potential appearing across a1predetermined portion of said field winding, and restraining meansenergized only with the induced alternating-current fiowing in saidfield winding.

3. In combination with a synchronous dynamoelectricv machine havingrelatively movable field and armature windings and means for controllingthe circuit of the armature winding on the occurrence of a turn-to-turnfault thereon including a circuit interrupter, an electroresponsivedevice for controlling the operation of said circuit interruptercomprising a movable member, a pair of motor elements for producingopposed torques on said movable element, means for energizing one ofsaid motor elements in accordance with the alternating potential appearing across a predetermined portion of said field winding, and means forenergizing said other motor element only in accordance with the inducedalternating current flowing in said field winding.

4. In combination with an alternating-current circuit, a synchronousdynamoelectric machine having relatively movable field and armaturewindings,means including an electric circuit interrupter forinterconnecting said alternatingcurrent circuit and said armaturewinding, a source of direct-current potential for energizing said fieldwinding, a micltap on said field winding, an electroresponsive devicefor controlling said circuit interrupter including a movable member, amotor element for causing an operating torque to be applied to saidmovable member, means for energizing said motor element with thealternating potential appearing across the midtap and one of theterminals of said field winding so as to isolate said armature of saiddynamoelectric machine from said alternating current circuit upon theoccurrence of a turn-to-turn fault on said armature winding, and meansresponsive to the induced alternating current flowing in said fieldwinding for preventing faulty operation of said electroresponsive deviceunder substantially all external fault conditions.

5. In a system for protecting against turn-toturn faults on the armaturewindings of synchronous dynamoelectric machines having relativelymovable field and armature windings, an electroresponsive device havinga plurality of windings, means for energizing one of said windings ofsaidelectroresponsive device in. accordancewith the alternatingpotential appearing across one-half of said field winding of saiddynamoelectric machine, and means for energizing said other winding ofsaid electroresponsive device in response to the inducedalternating-current flowing through said field winding to prevent faultyoperation of said electroresponsive device by virtue of dissymmetriesexisting in said dynamoelectric machine coupled with an external faultcondition.

6. In combination with an alternating-current circuit, a synchronousdynamoelectric machine 4 asaaaae across said third slip ring and one ofsaid other slip rings associated with said field winding, a second motorelement for restraining the operation of said circuit-controlling memberhaving a winding, and means for energizing said lastmentioned windingsolely in accordance with the alternating current induced in said fieldwinding so as to prevent operation of said electroresponsive deviceunder external fault conditions.

7. In combination with a synchronous dynamoelectric machine havingrelatively movable field and armature windings, an electroresponsivedevice including means for causing operation thereof in response to aturn-to-turn fault on said armature winding, means for energizing saidelectroresponsive device with the alternating potential appearing acrossone-half of said field winding so as to cause operation of saidelectroresponsive device by virtue of the relatively high potentialinduced in said field winding upon the occurrence of a fault'on saidarmature winding, restraining means for said electroresponsive device,and means for energizing said electroresponsive device from saiddynamoelectric machine to prevent dissymmetries in said dynamoelectricmachine from causing faulty operation thereof under substantially allexternal fault conditions.

8. In combination with asynchronous dynamoelectric machine havingrelatively movable field and armature windings, an electroresponsivedevice including means for causing operation thereof in response to aturn-to-turn fault on said armature winding, means for energizing saidelectroresponsive device with the alternating potential appearing acrossone-half of said field winding so as to cause operation of saidelectroresponsive device by virtue of the relatively high potentialinduced in said field winding upon the occurrence of a fault on saidarmature winding, restrainingmeans for said electroresponsive device,means for energizing said restraining means in response to thealternating current flowing in said field winding for preventing faultyoperation of said electroresponsive device under substantially allexternal fault conditions by virtue of dissymmetries in saiddynamoelectric machine, and means for preventing the direct-currentquantities of said field winding from substantially affecting theoperation of said electroresponsive device.

SELDEN B. CRARY.

